PHP Immutable class/property

immutable.php

<?php

immutable class Currency
{
  /** @var string */
  public $code; // this will change to readonly after constructor execution
  /** @var string */
  public $name; // this will change to readonly after constructor execution
  /** @var int */
  public $fractionDigits;
  
  public function __constructor(string $code, string $name, int $fractionDigits = 2)
  {
    $this->code = $code;
    $this->name = $name;
    $this->fractionDigits = $fractionDigits;
  } // after constructor execution all properties are marked as readonly
}

immutable class Money
{
  /** @var float */
  public $amount = 0; // this will change to readonly after constructor execution
  /** @var Currency */
  public $currency; // this will change to readonly after constructor execution
  
  public function __constructor(float $amount, Currency $currency)
  {
    $this->amount = $amount;
    $this->currency = $currency;
  } // after constructor execution all properties are marked as readonly also
  
  public function addAmount(float $amount) : static
  {
    return new static($this->amount + $amount, $this->currency);
  }
}

$currency = new Currency('PLN', 'Polish złoty');
$money = new Money(150, $currency);

readonly.php

<?php

class Invoice
{
  /** @var Money */
  public readonly $totalAmount; // this will change to readonly after assign
  
  public function __constructor(array $products)
  {
    $totalAmount = new Money(0, new Currency('PLN'));
    foreach ($products as $product) {
      $totalAmount = $totalAmount->addAmount($product->getPrice($totalAmount->currency));
    }
    $this->totalAmount = $totalAmount;
  }
}

$invoice = new Invoice($products);
$invoice->totalAmount; // this property is public but set as readonly and there is no way to change it